1. Field of the Invention
The present invention relates to a projector type headlamp for a vehicle. More particularly, the present invention relates to a headlamp for a vehicle that forms a low beam light distribution pattern.
2. Related Art
In general, a projector type headlamp for a vehicle is composed as follows. A projection lens is arranged on an optical axis extending in the longitudinal direction of the vehicle, and a light source is arranged at the rear of a rear side focus of the projection lens. Light emitted from this light source is reflected by a reflector and is converged toward an optical axis. In the case of a headlight for a vehicle which is used for producing a low beam, a portion of the reflected light sent from the reflector is shaded by a shade arranged so that an upper end edge of the shade is located at a position close to the rear side focus of the projection lens.
JP-A-2004-127830. (“JP '830”) describes a side insertion type projector type headlamp for a vehicle. In this type of headlamp, a linear light source extends in an axial direction of the light source bulb, which is inserted and fixed to the reflector from a side of the optical axis.
The headlamp for a vehicle described in JP-A-2005-100766 (“JP '766”) includes a first addition reflector, which is arranged between the light source bulb and the shade and reflects light that is sent from the light source toward a front region in the bulb insertion direction, and a second addition reflector, which is above the front region in the bulb insertion direction and reflects light that is reflected by the first addition reflector forward while converging the light toward the optical axis.
When the side insertion type lighting device structure described in JP '830 is employed, it is possible to make the lighting device compact by reducing a length in the longitudinal direction of the lighting device. However, because the light source is composed as a linear light source extending in the bulb axis direction, an amount of light incident upon a front region in the bulb insertion direction on the reflecting face of the reflector is extremely small. Accordingly, it is impossible to ensure a sufficient brightness of the light distribution pattern for a low beam. This is because the light distribution characteristics of the linear light source provide a high luminous intensity in the direction perpendicular to the bulb axis and a low luminous intensity in the direction of the bulb axis.
When the lighting device structure described in JP '766, in which the first and these second addition reflectors are provided, is employed, the light that is directly sent from the light source that would be shaded by the shade is made to be incident upon a projection lens by the first and the second addition reflectors. Thus, incident light can be effectively used as light for irradiating forward. In this case, according to the headlamp for a vehicle described in JP '766, a surface shape of the reflecting face of the first addition reflector is formed into an ellipsoid of revolution, the first focus of which is at a position of the light source and the second focus of which is at a position between the first addition reflector and the second addition reflector. Therefore, a surface shape of the second addition reflector can be set based on the assumption that a virtual light source is arranged at the second focus of the first addition reflector. Due to the foregoing, the light distribution can be easily controlled.
However, the following problems may be encountered. When the surface shape of the reflecting face of the first addition reflector is formed into a shape of an ellipsoid of revolution, radiation heat emitted from the light source is concentrated upon the second focus. Accordingly, the second addition reflector located close to this second focus tends to be heated to a high temperature. Therefore, an undercoat of a vapor-deposited film forming the reflecting face of the second addition reflector is quickly deteriorated by an influence of the radiation heat emitted from the light source.
The present invention has been designed in view of the above circumstances.